1. Field of the Invention
The present invention relates to electric power steering systems in vehicles for driving a motor based on a corrected motor control signal obtained by correcting, according to an amount of motor control signal correction, the motor control signal calculated from the steering torque.
2. Description of the Related Art
There is a known technique for enhancing turning performance of a vehicle where the ratio by which the engine driving force is variably distributed between the right and left driven wheels, and specifically the driving force distributed to the outer turning wheel is increased while the driving force distributed to the inner turning wheel is decreased so as to generate a yaw moment in the turning direction. In a vehicle having such a driving force distribution device, there is a disadvantage that when the driving forces distributed to the right and left driven wheels are varied, an undesirable steering force is produced in the right and left driven wheels which also serve as steered wheels (the torque steer phenomenon). Moreover, the present assignee has already made a proposal in which the torque steer phenomenon is lessened by utilizing an electric power steering system, which is provided in a vehicle and generates a steering assist torque in the electric power steering system so as to counteract the above-mentioned undesirable steering force (ref. Japanese Patent Application No. 11-63114).
In the above-mentioned system previously proposed by the assignee, the control means for controlling the driving force distribution device calculates an amount of motor control signal correction required for counteracting the torque steer phenomenon according to the distribution of the driving force, and the control means of the electric power steering system drives a motor based on the result of correcting the motor control signal by means of the above-mentioned amount of motor control signal correction. Therefore, in the case where the control means for controlling the driving force distribution device malfunctions, and so outputs an abnormal amount of motor control signal correction, there is a possibility that the motor could generate an inadequate steering assist torque, so giving an disagreeable sensation to the driver. In particular, since the effect of steering becomes large when a vehicle is travelling at high speed, the disagreeable sensation experienced by the driver also becomes strong at such speed.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an objective of the present invention to lessen the disagreeable sensation experienced by the driver when the amount of motor control signal correction required for correcting the motor control signal in the electric power steering system becomes abnormal.
In order to achieve the above-mentioned objective, in accordance with the invention, an electric power steering system is proposed which comprises: a motor control signal calculation means for calculating a motor control signal for driving a motor to generate a steering assist torque, based on at least a steering torque detected by a steering torque detecting means; a motor control signal correction calculation means for calculating a corrected motor control signal from an amount of motor control signal correction calculated by an external control means and said motor control signal; a driving means for driving the motor based on the corrected motor control signal calculated by the motor control signal correction calculation means; and a motor control signal correction amount reducing means for reducing said amount of motor control signal correction according to an increase in a vehicle speed.
In accordance with the above-mentioned arrangement of the invention, since the motor is driven according to a corrected motor control signal which has been obtained by correcting the motor control signal by means of an amount of motor control signal correction calculated by an external control means, when the above-mentioned external control means malfunctions and the amount of motor control signal correction becomes abnormal, the corrected motor control signal also becomes abnormal and it becomes impossible for the electric power steering system to generate an adequate steering assist torque. In particular, since the effect of steering becomes large when a vehicle is driven at high speed, if an inadequate steering assist torque is generated, the disagreeable sensation experienced by the driver could become strong. However, since the motor control signal correction amount reducing means of the invention reduces the amount of motor control signal correction according to an increase in the vehicle speed, the influence of an abnormal amount of motor control signal correction can be lessened so lessening the disagreeable sensation experienced by the driver.
The target current IMS in the embodiments corresponds to the motor control signal of the present invention, the corrected target current IMSxe2x80x2 in the embodiments corresponds to the corrected motor control signal of the present invention, the amount of current correction xcex94I in the embodiments corresponds to the amount of motor control signal correction of the present invention, the target current setting means M9 in the embodiments corresponds to the motor control signal calculating means of the present invention, the current correction amount reducing means M15 in the embodiments corresponds to the motor control signal correction amount reducing means of the present invention, the first electronic control unit U1 in the embodiments corresponds to the external control means of the present invention, the motor driver 32 in the embodiments corresponds to the driving means of the present invention, and the subtraction means 33 in the embodiments corresponds to the corrected motor control signal calculating means of the present invention.